Laminate board containing uniformly distributed filler particles and method for producing the same

ABSTRACT

A copper clad laminate board for a printed circuit has a center plate which is a thermosetting resin containing hollow glass microspheres uniformly distributed in the resin. The center plate has sheets provided on both sides thereof, which sheets are a thermosetting resin having a reinforcing fabric. The board has a high electrical resistance and resists bending during subsequent processing.

This is a division of application Ser. No. 765,476 filed Aug. 14, 1985now U.S. Pat. No. 4,661,301 issued Apr. 28, 1987.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention conerns a laminate board which is applicable tothe manufacture of a printed circuit board and a process for continuousmolding of a plate for use in producing the laminate board.

2. Description of the Prior Art

It is well-known that a material having a low dielectric constant (Er)and dissipation factor (tan δ) is advantageous for use in printedcircuit boards. As the dielectric constant goes down, the electricperformance, as measured by time delay, capacitance, and other factors,is improved.

Many materials having a low dielectric constant, such as polyethylene,poly 4-methyl pentene-1, polystyrene, polytetrafluoroethylene are notnecessarily suitable for use in printed circuit boards from theviewpoints of heat resistance, cost or processability.

Incorporation of a filler into a resin is a well-known technique forimproving properties of the resin such as electric, optical, mechanical,electromagnetic, frictional, chemical and other properties.

One method for lowering the dielectric constant of a material is toincorporate a filler such as one consisting of hollow microspheres, asdisclosed in Japanese Patent Publication No. 18353/82 (Tokko-sho57-18353); and F. W. Haining & D. G. Herbaugh, IBM TECH. DISCLOSURE BULL(USA) Vol. 22. No. 5, 1799, October 1979. The glass epoxy laminatesprepared by conventional processes according to the above method usingepoxy/microsphere prepregs, however, do not necessarily perform well ina printed circuit board. For example, the board's deviation fromflatness, such as bending and twisting causes problems in subsequenttreatments such as soldering, etching, drilling, laminating of resist,and plating. These defects may be due to uneven distribution of thefiller in the prepregs. Furthermore, the electrical insulation of theboard is not as large as is required when the board is immersed inboiling water. Voids formed between the filler and the reinforcingfabric are considered responsible for these disadvantages.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a novel laminate boardwhich satisfies the requirements of electrical insulation and lowdielectric constant. Another object is to provide a new process forproducing a plate applicable to the production of a laminate with noneof the disadvantages described above. More specifically, the inventionprovides a laminate board comprising a center plate which comprises aresin and filler particles uniformly distributed in said resin andsheets provided on both sides of the center plate which comprise resinand reinforcing fabric. When the board is used in a printed circuitboard, copper foils are further laminated to it.

The plate of this invention is prepared by continuous molding of amixture comprising a resin and a filler, wherein the mixture of resinand filler is at least partly solidified while moving through avertically disposed pass.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-section of an embodiment of the laminate boardaccording to the present invention. FIG. 2 shows a comparativecross-section of a laminate board disclosed in the prior art (JapanesePatent Publication No. 18353/82).

FIG. 3 shows a schematic diagram of the process of this invention.

FIG. 4 shows a microphotograph of a cross-section of a laminate board ofthis invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A typical example of a copper foil-clad laminate board of this inventionis shown in FIG. 1. A center plate is sandwiched between impregnatedreinforcing fabrics 2 which are covered by copper foils 1. The centerplate comprises a resin 3 wherein particles of a filler 4 aredistributed uniformly.

It is well-known to prepare a continuous plate of a comparatively largethickness by extruding a monomer or polymer from a slit or die, allowingthe extruded material to flow on a horizontally moving belt andhardening it by heating or cooling during its passage on the belt.

Such method is satisfactory for the preparation of a plate fromhomogeneous resin liquid such as molten resin or a solution of a resin.However, in the case of a heterogeneous solution like polymer/fillermixture the known extrusion method is not satisfactory. In order toprepare a uniform mixture, it is necessary to use a resin having as lowa viscosity as possible. However, the low viscosity of the mixture (lessthan 10³ poises) together with the specific gravity difference (of morethan 0.2) between the resin and the filler give rise to seriousdislocation of the filler during continuous molding of the plate.Consequently, as shown in FIG. 2, asymmetric distribution of the filler8 in the resin 7 occurs in the direction perpendicular to the surface ofthe plate (hereafter designated as Z-direction) in each layer betweenneighboring glass cloths 6. The use of this plate in a printed circuitboard clad with copper foils 5 imparts undesirable properties to theboard, in particular, bending of the board. This bending is oftenaggravated by subsequent treatments such as soldering and etching.

To prevent asymmetric filler distribution in a plate extruded by theprior art method, it is necessary to reduce the thickness of the layerof resin/filler mixture in order to bring the filler in close contactwith the reinforcing fabric 2 and thereby prevent movement of the fillerin the resin. This means that the adjacent reinforcing fabrics must bebrought into contact with each other and, accordingly, only a smallamount of filler can be used. Furthermore, because in practice theparticle size distribution of the filler is very large, it is notpossible to prevent movement of the smaller filler particles.

FIG. 3 shows a schematic diagram of the process of this invention. Amixture 9 of a resin and a filler is extruded from a slit 10 to flowonto vertical double belt press 11. By this arrangement, the filler doesnot move in the horizontal direction (Z-direction) even when thedifference in specific gravity between the resin and the filler islarge. The mixture is hardened by heating or cooling while it is passingbetween the pair of belts. The plate, which is partially hardened butretains some plasticity, is conveyed by roll 12 which changes thedirection of the movement of the plate from vertical to horizontal. Inorder to prevent the plate from breaking upon being bent around theroll, the size of the roll is selected according to the hardness of theplate. Thus, the harder the plate, the larger the roll. Upon leaving theroll 12, the plate is conveyed on a horizontal double belt press 13. Atthis point, the filler neither sinks nor floats to cause asymmetricdistribution, since the resin has hardened sufficiently to preventmovement of the filler particles.

In the present invention, thermosetting resins such as phenol, epoxy,furane, unsaturated polyester, xylene, alkyd, sulfonic amide, melamineresins, and thermoplastic resins such as polyamide, polyester,polyolefin, polystyrene, polyphenylene oxide, polyphenylene sulfide,polysulfone, polyethersulfone, polyetherimide, polyacetal,polycarbonate, polyvinylidene fluoride and polyacrylate resins may beused. The resin of the center plate is not necessarily the same as thatof the sheets to be laminated on the center plate.

The filler material is selected according to the required properties ofthe board. For example, for the purpose of lowering the dielectricconstant, it is advantageous to use hollow microspheres of alumina,silica, zirconia, glass, carbon and phenol resin. The preferred fillerin this case consists of hollow glass microspheres of 20-150 μm indiameter having a glass thickness 0.5-2 micro meters. The volumefraction of filler based on the total volume of the center plate shouldbe 0.3 to 0.8, more preferably 0.5 to 0.7.

As the reinforcing fabric, woven and non-woven sheets made of inorganicor organic fibers, preferably glass fiber, quartz fiber, polyaramidfiber "Kevlar" may be used.

This invention is particularly useful in situations where the differencein specific gravity between the resin and the filler is large,especially more than 0.5, were the viscosity of the resin to be mixed islow, especially less than 10² poises, and further, where the size of thefiller particles is relatively large (more than 5 microns). For example,in practical applications, there are many cases where a resin must beused which has a viscosity of less than 10 poise.

EXAMPLE 1

Epoxy resin composition was prepared with 100 parts of diglycidyl etherof Bisphenol A ("EPIKOTE" 828: YUKA SHELL CO.), 86 parts of methylhexahydrophthalic anhydride ("EPICLON" B650: DAINIHON INK CO.) and 1part of dimethylbenzylamine (Parts refer to parts by weight). This epoxyresin composition has a viscosity of 5 poises at 25° C.

Glass microsphere ("GLASS BUBBLES" B38/4000: 3M INC.) of 50 micro metersin average diameter and specific gravity 0.38 was used as a filler.

A mixture was obtained by mixing the epoxy resin composition and theglass microspheres in a volume ratio of 40/60 at room temperature. Themixture was allowed to defoam in vacuum. Then it was extruded from aslit onto a vertically disposed moving double belt press of 2 meterlong, as shown in FIG. 3. The speed of the pair of moving belts was 8meters/hour. The temperature of the belt was kept at 150° C. Afterpassing through the vertically disposed belt, the almost fully hardenedplate was moved along a roll of 900 mm diameter to change its directionfrom vertical to horizontal. Then, the plate was moved horizontally by asecond double belt press of 2 meter long kept at a temperature of 170°C. As a result, a plate of 15 mm in thickness and of specific gravity0.80 was obtained.

Glass cloth (WE116E: NITTO BOSEKI CO.) of 150 μm thickness wasimpregnated with the above epoxy resin composition to obtain a sheet(prepreg).

A laminate board was prepared by sandwiching the plate between twosheets of prepreg and then between two sheets of copper foil as in FIG.1 by use of a conventional heating press.

The properties of the board are shown in Table 1 from which it is seenthat a laminate board of good performance was obtained.

                  TABLE 1                                                         ______________________________________                                        Properties of board                                                                                Comp.     Comp.                                                       Example 1                                                                             Ex. 1     Ex. 2                                          ______________________________________                                        Thickness (mm) 1.60      1.63      1.60                                       Er (1 MHz)     2.5       2.6       2.7                                        tan δ (1 MHz)                                                                          0.010     0.015     0.020                                      Insulation Resistance (Ω)                                                              5.6 × 10.sup.9                                                                    1.5 × 10.sup.7                                                                    3.5 × 10.sup.9                       Bending (%)    0.0       0.7       1.5                                        ______________________________________                                         Index of bending was evaluated according to the standard test method.         (IPCTM-650).                                                             

COMPARATIVE EXAMPLE 1

Using the same materials (epoxy resin composition, hollow microsphereand glass cloth) as in Example 1, a prepreg was produced by ordinaryimpregnation according to a prior art method (Tokko-sho 57-18353). Aboard was obtained by laminating six sheets of the prepreg and twosheets of copper foil by use of heating press. FIG. 2 shows theschematic diagram of the board obtained. Table 1 shows the properties ofthis board as compared with the board of Example 1. It can be seen thatthe electrical insulation resistance of this board is small and itsbending is a little larger than observed in the board of Example 1.

COMPARATIVE EXAMPLE 2

Using the same materials as in Example 1, the epoxy resin compositionand glass hollow microspheres were mixed. The mixture was extruded froma slit onto a horizontal double belt press. The speed of the moving beltwas 8 meters/hr and its temperature was kept at 150° C.

A copper-clad laminate board was obtained in the same manner as inExample 1. The properties of this board are shown in the third column ofTable 1. It can be seen that the bending of this board is considerablylarger than was observed in Example 1.

We claim:
 1. A copper clad laminate board for a printed circuit, havinga thickness from 0.5 to 1.6 mm, comprising a center plate which consistsof a thermosetting resin and hollow glass microspheres, uniformlydistributed in said resin, the volume fraction of which ranges from 0.5to 0.7, based upon the volume of the center plate, and sheets, providedon both sides of said center plate, which comprise a thermosetting resinand a reinforcing fabric, the specific gravity of the laminate boardbeing from 0.8 to 1.3.
 2. The laminate board of claim 1, wherein thecenter plate has a thickness of at least 0.3 mm.
 3. The laminate boardof claim 1, wherein the reinforcing fabric is made of glass.